1. Field of the Invention
The present invention relates to a focal point detection apparatus, and, more particularly to an improvement in a charge storage function and charge transfer control function of a light sensing portion for detecting the focal point, the light sensing portion being a component of a focal point detection apparatus having a plurality of focal point detection areas.
2. Related Background Art
Hitherto, there has been available a focal point detection apparatus having a plurality of focal point detection areas on an image photographing surface and acting to detect a focal point in accordance with output signals transmitted from a plurality of light sensing portions which correspond to a plurality of the focal point detection areas.
An apparatus of the type described above performs the charge storage and transfer to and from its plurality of light sensing portions as follows because its microcomputer for calculating signals transmitted from the light sensing portion cannot simultaneously process the output signals transmitted from a plurality of the light sensing portions:
(1) The charge storages in all of the light sensing portions are simultaneously commenced and the charge of a light sensing portion which has been stored is temporarily stored in a memory until the charges are stored in all of the light sensing portions. When the charges have been stored in all of the light sensing portions, the charges stored in all of the pixels are collectively and in parallel transferred to a CCD shift register before they are transferred from the CCD shift register to an external circuit in a serial manner (refer to Japanese Patent Application Laid-open No. 62-169569). PA1 (2) The charge storages in all of the light sensing portions are simultaneously commenced and the stored charges are sequentially transferred from the light sensing portion in which the charge has been stored (refer to Japanese Patent Application Laid-open No. 2-64516). PA1 (3) In order to prevent overlap of the charge storages and transferences of charges in a plurality of light sensing portions in terms of time, a process is performed in a time sequential manner. PA1 (1) The charge storages in a plurality of pairs of light storage portions are simultaneously completed and the charges of the other pair of the light sensing portions are temporarily stored in the memory during the transference of the charges stored in a pair of the light sensing portions provided with the common shift register. Therefore, the S/N ratio of the signal will be deteriorated due to a mixture of noise such as a dark current with a signal of the latter light sensing portion. PA1 (2) In order to overcome the above-described problem, it might be considered feasible to employ a structure in which a common shift register is provided for the latter pair of the light sensing portions while detouring the former pair of the light sensing portions. However, the length of the shift register cannot be reduced due to detouring, causing noise to be mixed during the transference. As a result, the S/N ratio of the signal will be deteriorated and the charge transfer time cannot be shortened.
However, the above-described conventional apparatus (1) encounters a problem of a deterioration in the S/N ratio of a signal due to a mixture of noise such as a dark current because the charge stored in a light sensing portion is temporarily stored in a memory until the charges are stored in all of the light sensing portions.
The conventional apparatus (2) encounters a similar problem to that experienced with the conventional apparatus (1) because, if the charge stored in the other light sensing portion is being transferred at the time of the completion of the charge storage, the charge must be temporarily stored in a memory in a period of the above-described transference operation.
In addition, the conventional apparatus (3) suffers from a problem in that the focal point cannot be detected quickly because the charge storage and the transference of the charge stored in each light sensing portion are processed in the time sequential manner even if the subject displays low brightness in which the charge storage time becomes long.
There has been available a focal point detection apparatus which is arranged in such a manner that two focal point detection areas are provided on an image photographing surface to intersect each other and which performs the focal point detection calculations in response to output signals from two pairs of light sensing portions provided to correspond to the above-described focal point detection areas (refer to Japanese Patent Application Laid-open No. 2-134609).
In a focal point detection apparatus of the type described above, light sensing portions for performing the photoelectrical conversion and transfer portions (hereinafter called "shift registers") for transferring the stored charges are formed on one chip. Furthermore, the shift register for a pair of light sensing portion of two pairs of light sensing portions is commonly provided through the other pair of the light sensing portions. Therefore, no common shift register can be provided from another pair of the light sensing portions. Therefore, individual shift registers are provided for the light sensing portions.
The charge storages in the two pairs of the light sensing portions are controlled to be completed simultaneously. Furthermore, the transference of the charges stored in the pair of the light sensing portions provided with the common shift register is commenced and the charges stored in the other pair of light sensing portions are transferred to the memories individually provided for the light sensing portions so as to be temporarily stored. After the charges stored in the pair of the light sensing portions provided with the common shift register have been transferred, the charge of either one of the other pair of the light sensing portions stored in the memory is transferred. Then, the charge of the other light sensing portion is transferred after the above-described charge transference has been completed.
However, the above-described conventional apparatus encounters the following problems: